Investigating the Nature of Urban Life

In Phoenix, a small army of researchers is conducting the nation's first long-term ecological study of a city--and making some surprising discoveries in the process

10-01-2006
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Mark Wexler

WHILE MOST SCIENTISTS prefer to study wildlife in remote, pristine places far from the influences of people, Paige Warren opts for working right in the heart of human habitat. "You never know what kinds of interesting encounters you'll have," says the biologist, who has been questioned by police officers, cornered by chatty homeowners and watched by wary coyotes while investigating bird behavior in the residential neighborhoods and parks of Phoenix, Arizona. "The challenges are definitely unique."

Warren is part of a small army of researchers who are trying to learn everything they can about the effects of development, climate change, landscaping and other factors on the wild creatures that live in the Valley of the Sun, as the greater Phoenix metropolitan area is known. Headquartered at Arizona State University (ASU), the National Science Foundation-funded project they are participating in represents the country's first-ever long-term study of what the researchers call a "human-dominated ecosystem."

"When we began this work in 1997, we decided we need to use the same scientific procedures to answer questions about an urban area that we use to study a wilderness location," says ASU ecologist Nancy Grimm, codirector of the Central Arizona-Phoenix Long-Term Ecological Research Project. Nearly a decade later, Grimm and the 100 or so biologists, geographers, social scientists and technicians taking part in the 13-year study are certain they are looking at a distinct, fully functioning ecosystem that has received far too little attention from natural scientists in the past.

With nearly three-fourths of the U.S. population now living in the nation's burgeoning metropolitan areas, observes Grimm, "it's imperative that we figure out how to maintain and improve the ecological health of the places where most Americans make their homes." The researchers believe there is nothing contradictory about the terms "nature" and "city." In fact, they've discovered that the overall abundance of birds and other organisms is much greater in Phoenix than in the surrounding desert.

One reason: Residents have created a sort of artificial oasis in the nation's fifth largest city. The municipal district and suburbs are dotted with canals, ponds, swimming pools (at last count, more than a half million) and dozens of parks and greenways. The community of Scottsdale alone now has nearly 170 small lakes, compared to zero in the mid-1900s. Yet in many respects, this desert oasis is not a Garden of Eden. All of the impounded water attracts a lot of mosquitoes, creatures normally not common in such an arid climate, and much of the area is landscaped with exotic vegetation that is displacing native flora.

"Homeowners can buy a variety of drought-tolerant plants at local nurseries," says Grimm, "but our studies have found that if those plants are not native to this region, they're not going to support our declining pollinators." Providing habitat for native pollinators, she notes, is vital to ensuring biodiversity.

Curiously, the scientists have discovered that more species of native birds prefer to live and breed in the city's wealthy neighborhoods than in middle- and lower-income areas. Warren and fellow biologists Ann Kinzig and Chris Martin came to that conclusion after measuring the abundance and diversity of birds in 16 parks in different parts of Phoenix. They found that the upper-income neighborhoods have, on average, nearly twice as many species as the low-income areas. "You'd think that just the opposite should be the case because the parks in the lower-income communities have more mature trees," says Warren, who has found similar results in neighborhoods in Baltimore, Maryland, site of a second National Science Foundation long-term urban study.

In fact, the birds seem to be attracted by the overall quality of habitat in a neighborhood around a park, not by the park itself. Higher-income communities in Phoenix, notes the biologist, tend to have more cacti and other desert plants--and more plant diversity in general--than the other residential areas, which make them so appealing to the animals.

As might be expected, the researchers have found that bird diversity is low in the noisier parts of the city. To successfully stake out territories and attract mates in such environments, some species, such as the blue-throated hummingbird, simply sing louder--a reflexive response, like people raising their voices in noisy restaurants.

"We still have so much to learn about the abilities and habitat needs of the wild animals living in our midst," says Warren, who these days spends more of her time analyzing data in her office at the University of Massachusetts than collecting information in the field. "We need to understand how to live sustainably with wildlife in our cities," she adds. "Otherwise, we won't have diverse wildlife and our communities will be much less vital places to live in."

Coping With the Sounds of CitiesFinding mates in noisy places can be difficult, but some songbirds have evolved unique adaptations to cope with urban environments. In the first known case of a species changing the pitch of its songs, Dutch researchers found recently that great tits (relatives of North American chickadees) sing at high frequency levels near busy roads in the European cities where they range. In doing so, the tits can be heard by other birds above the low-frequency hum of the traffic. In quieter neighborhoods, the tits croon using notes lower on the scale. In another study, Emory University scientist Donna Maney discovered that female white-throated sparrows process sounds differently during breeding season. "Our work suggests that estrogen levels, which are normally high only when it's time to reproduce, may actually alter auditory pathways," says Maney. This helps the female sparrows block out the surrounding racket and focus in on the songs of male sparrows.